The excellent molding workability, mechanical properties and aesthetic qualities of synthetic resins have led to their wide use and significant importance in molded articles, films, fibers and coating materials. Because synthetic resins are generally flammable, they must be treated to impart flame retardance when used in most electrical and electronic devices, automobiles and the like. Flame retardant agents are commonly used to impart flame retardance to synthetic resins, and known flame retardant agents include organic halogen compounds, halogen-containing organic phosphorus compounds, antimony compounds, inorganic hydroxides, organic phosphorus compounds and the like.
The currently popular halogen compounds such as organic halogen compounds and halogen-containing organic phosphorus compounds are associated with risks of hydrogen halide generation, dioxin production and die corrosion during the processes of heating, melting and incineration in the step of molding synthetic resins, and therefore demand is rising for halogen-free flame retardant agents.
Antimony compounds are also commonly used in combination with halogen compounds, but the harmfulness of antimony compounds themselves has tended to limit their use.
Methods which avoid the use of halogen-based flame retardants employ inorganic hydroxides in their place, but because inorganic hydroxides exhibit flame retardance by the water produced by thermal decomposition, the flame retardant effect is low and large amounts thereof must therefore be added. The result of such large addition often impairs the original properties of the resin.
Another approach to avoiding the use of halogen-based flame retardants is to use organic phosphorus compounds such as triphenyl phosphate (TPP) and tricresyl phosphate (TCP), but such organic phosphorus compounds are unsatisfactory from the standpoint of volatility, sublimation, moisture resistance and heat resistance. In addition, these organic phosphorus compounds are phosphoric acid ester-based flame retardants, and therefore when heated and kneaded with synthetic resins such as polyesters they undergo transesterification whereby the molecular weight of the synthetic resin is significantly lowered and the original properties of the synthetic resin are reduced. Moreover, phosphoric acid ester-based flame retardants themselves gradually undergo hydrolysis by moisture in the air which can lead to production of phosphoric acid, and production of phosphoric acid in a synthetic resin lowers the molecular weight of the synthetic resin and can potentially cause shorts when the resin is used for electrical or electronic applications.
In order to solve these problems of transesterification or hydrolysis when using phosphoric acid ester-based flame retardants, the present applicant has developed the flame retardant thermoplastic resin composition described in Japanese Unexamined Patent Publication No. 2001-294759. However, the organic phosphorus compounds which are actually used in this type of flame-retardant thermoplastic resin composition all have phenolic hydroxyl groups and therefore are not sufficiently inactive with respect to the synthetic resin, while the reduced heat and moisture resistance of the obtained flame-retardant thermoplastic resin composition is another unacceptable problem.
The present applicant has also disclosed, in Japanese Unexamined Patent Publication No. 2002-275473, that flame-retardant fibers with excellent wash resistance can be obtained by adding an organic phosphorus compound-containing flame-retardant coating agent to a fiber material by a post-processing method. However, this publication only describes adding the organic phosphorus compound-containing flame-retardant coating agent to a fibrous material by post-processing, while nothing is mentioned or suggested in regard to including the organic phosphorus compound in the synthetic resin, or the effect thereof on resin materials. In addition, addition of an organic phosphorus compound to a fiber material by post-processing in this manner is still inadequate from the standpoint of uptake efficiency.